Inkjet printing mechanisms use cartridges, often called "pens," which eject drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, ejecting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a "service station" mechanism is supported by the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which substantially seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as "spitting," with the waste ink being collected in a "spittoon" reservoir portion of the service station. After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead. The wiping action is usually achieved through relative motion of the printhead and wiper, for instance by moving the printhead across the wiper, by moving the wiper across the printhead, or by moving both the printhead and the wiper.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast printing with darker blacks and more vivid colors, pigment-based inks have been developed. These pigment-based inks have a higher solid content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to form high quality images on readily available and economical plain paper.
Typically, these inks are supplied in a reservoir housed by the inkjet cartridge, so when the pen is emptied, the entire cartridge is replaced. Some cartridges are monochrome (single color), for instance, carrying only black ink, while other cartridges are multi-color, typically carrying cyan, magenta and yellow inks. Some printing mechanisms use four monochrome cartridges, while others use a black monochrome cartridge in combination with a tri-color cartridge. Recently, an imaging cartridge system has been introduced by the Hewlett-Packard Company of Palo Alto, Calif., as the DeskJet.RTM. 693C model inkjet printer. This is a two-pen printer which uses a tri-color pen, carrying full dye-loads of cyan, magenta and yellow, and a black cartridge which may be replaced with a tri-color imaging cartridge. This imaging cartridge carries reduced dye-load concentrations of some colors, such as cyan and magenta, along with a full or partial dye-load concentration of black ink. The imaging cartridge allows the printer to produce more continuous tone changes, particularly flesh tones, so the resulting image has near-photographic quality, with very little graininess.
As the inkjet industry investigates new printhead designs, the tendency is toward using permanent or semi-permanent printheads in what is known in the industry as an "off-axis" printer. In an off-axis system, the printheads carry only a small ink supply across the printzone, with this supply being replenished, for example through tubing that delivers ink from an "off-axis" main reservoir placed at a remote, stationary location within the printer. Rather than purchasing an entire new cartridge which includes a costly new printhead, the consumer buys only a new supply of ink for the main reservoir. Typically, the fresh ink supplies are sold individually by color, although in some implementations, a multi-color supply may be furnished.
Both the multi-color and monochrome replacement schemes have advantages and disadvantages, whether they are offered as a replaceable cartridge, or as an off-axis supply. The combined multi-color system provides both the purchaser and the store with a single consumable item which is easily ordered, stocked, and replaced. The disadvantage of the multi-color modules is that some ink is always wasted because one color typically runs out before the others, resulting in a higher cost per page of printing for the consumer. The individual single color system solves the wasted ink problem by replacing only the empty color, but unfortunately, both the purchaser and the store must separately stock each color to ensure that the correct color ink is available when needed. This separate stocking issue has been compounded from the basic four colors (black, cyan, magenta and yellow) through the introduction of the imaging cartridge system, which also uses reduced dye-loads to produce multiple shades of cyan, magenta, yellow, or gray, or other formulations, such as red, green and blue inks, or perhaps other custom blended colors.
To obtain a full color gamut or a particular color hue, at some point from ink formulation to viewing of the final image, the desired colorants are mixed. Indeed, this whole issue of multiple shades of color, as well as custom blended colors, is one which has been addressed in varying degrees by the inkjet industry. In the earlier scenarios, four colors were mixed at the factory, specifically black, cyan, magenta and yellow. This system increased the color gamut available by using the eye to blend these basic colors into varying degrees of perceived shades and tones. To obtain better print quality and blending of shades, the printheads were then improved to deposit smaller droplets on a page, yielding more drops per unit area with more detail appearing in the resulting image. The use of the imaging cartridge which has various degrees of dye loads changed the amount of color deposited per drop by changing the saturation level of the colorant. One inkjet manufacturer has even mixed ink concentrations at the printhead, by adjusting the colorant concentration within the nozzle firing chamber, as described more fully in U.S. Pat. No. 5,371,529 and European Patent Application No. 0 655 337 A2. In this "blending in the printhead" system, the precise hue is ejected from the printhead, so the eye no longer needs to mix a group of colors to perceive the desired hue. Rather than using an elaborate printhead control scheme, it may be more desirable to blend a desired hue at the factory, and ship it directly to the customer. Such custom-blended colors may be particularly useful for industries where a particular color forms a portion of their trademark, or in other applications where particular colors are repeatedly used.
Another drawback of both the current multi-color and monochrome replacement schemes is that these inks often have a "shelf life." Currently, the ink compositions are pre-mixed by the manufacturer, and they must retain their composition and consistency from the time of manufacture until the time of use. Unfortunately, today's inks have a limited shelf life after which the compositions begin to decay, producing less than optimal print quality. Varying environmental conditions encountered during transport and storage may also adversely effect the more delicate ink constituents, leading to degraded print quality.
Thus, it would be desirable to provide a new system for supplying ink to consumers which avoids the wasted ink issue of the multi-color supplies, while also avoiding the stocking issues of the single-color supplies. It would also be desirable for such a new system to provide ink compositions with extended shelf lives to produce images of optimal quality, as well as to provide ink compositions with custom-blended colors.